Net torque from tangential forces on a disc

Steps:

• Torque for each force is τ = F * R * sinθ, where θ is the angle between the position vector and force direction; for tangential forces, sinθ = 1, so τ = FR per force.
• Calculate net torque by vector summing individual torques about center X, considering direction (clockwise or counterclockwise).
• For total magnitude 2FR, the arrangement must have two effective FR torques in the same rotational direction without cancellation.
• Evaluate each option's force directions to find the net torque.

Why A is correct:

• Two tangential forces on opposite sides act in the same rotational sense, each contributing FR torque that adds up to 2FR.

Why the others are wrong:

• B: All forces radial, so sinθ = 0, net torque = 0.
• C: Two tangential forces on opposite sides act in opposite senses, torques cancel to 0.
• D: Four forces symmetrically placed, torques pairwise cancel to 0.

Final answer: A